1. Field of the Invention
The present invention relates to a radio terminal that realizes power-save operations by intermittently activating a radio communication module for realizing radio communication with a radio base station; and more particularly to an end-to-end delay control method for suppressing the end-to-end delay time, which is the delay time required for end-to-end (inter-terminal) transmission and reception, to a stipulated standard value or less.
2. Description of the Related Art
In the radio packet communication mode that is stipulated in, for example, IEEE802.11, a power-save mode is established in which a receiver is periodically activated (awakened) and packets are transmitted and received. An outline of the operation of the power-save mode in IEEE802.11 is next briefly described with reference to FIG. 1.
A radio base station broadcasts beacon packets to all radio terminals at a fixed period, and all radio terminals that are associated with the radio base stations can receive these beacon packets.
A radio terminal that wishes to reduce power consumption shifts to a power-save mode in which the radio terminal is activated to transmit and receive radio waves only when a radio base station transmits the beacon and at other times shifts to a sleep state in which power is not supplied to the radio communication module and radio waves are not transmitted or received. Radio terminals that are in the sleep state are not able to transmit or receive packets, and the radio base station therefore determines which radio terminals are effecting power-save operations and stores packets that are addressed to these radio terminals that are in the power-save mode. The radio base station then uses the periodically transmitted beacon packets to report to each radio terminal which radio terminals currently have packets that are stored for them.
A radio terminal that receives notification of the arrival of packets by means of a beacon packet from the radio base station immediately responds to this notification of arrival and notifies the radio base station that it can now receive the packets. The radio base station, having received this response from the radio terminal, immediately transfers the packets that have been saved.
Even though active only when beacon packets are transmitted, a radio terminal that realizes power-save operations in this way is able to receive packets for data without any losses. In the following explanation, the time interval over which power is supplied and cut off to the radio communication module is referred to as the “activation period”.
In addition, a radio terminal that is effecting power-save operations can skip activation (supply of power) in any beacon period. In FIG. 1, radio terminal A is activated in each period, but radio terminal B is activated in every other period, whereby power consumption can be even more greatly limited than when activating with each period.
The disadvantage of the power-save mode operation is the delay in the transfer of packets. In other words, packets are stored in a radio base station during the interval from the arrival of the packets in the radio base station to the arrival of the activation period of the radio terminal to which these packets are addressed, and this operation therefore produces a maximum delay that is equal to the activation period of the destination radio terminal. In other words, the greater the power-save effect that a radio terminal obtains, the greater the delay time or jitter (fluctuation in delay time) in the radio space.
A service provider that offers voice service using, for example, VoIP (Voice-over Internet Protocol) on an IP network must meet speech quality standards that have been set by the Ministry of Public Management, Home Affairs, Posts and Telecommunications. Classes A–C are currently set as three levels of standards for rating the quality of VoIP, and the standard values shown in FIG. 2 must be met for comprehensive transmission quality rate (R value), end-to-end delay, and loss probability. In other words, the end-to-end delay time is stipulated to be less than 100 ms for Class A, less than 150 ms for Class B, and less than 400 ms for Class C. In this case, the end-to-end delay time is the delay time required for transmission and reception from end to end (between terminals).
However, a radio terminal that is operating in a power-save mode such as the mode described in the foregoing explanation cuts power consumption by means of periodic activation, and when attempting a conversation by VoIP in the power-save mode, the added delay in the radio space may exceed the stipulated quality limits.
On the other hand, preventing the power-save operation in the radio terminal, while minimizing the delay time in the radio space, may prevent the radio terminal from obtaining a practical conversation time, particularly when the radio terminal is a battery-driven mobile terminal.
As end-to-end delay control methods of the prior art for limiting end-to-end delay to less than a standard value, Japanese Patent Laid-Open Publication No. 2002-204278 and Japanese Patent Laid-Open Publication No. 2002-135330 disclose methods in which the delay time is measured in a transmission path in a data transmission system and the bit rate and communication bandwidth settings are modified when these values do not satisfy the standard value. However, in either case of the prior art, there is no radio space in the transmission path, and the use of these examples of the prior art therefore does not enable a limitation of the radio space delay that is caused by the power-save operations in radio terminals as described in the foregoing explanation.